You can't read too much into either of those numbers. 8 minutes early is calculated at the gate....he may well have taken off on the planned time. But why was he 21 late. If held well below his planned altitudes, then you'd slow down a bit as well. A couple of thousand feet low, and slightly late, would likely be almost the same fuel burn. But, looking at FR24, his altitudes look to be pretty normal all the way across, in which case he's lost 21 minutes...so a third of an hours extra fuel burn. I'd guess that amount would be about 2,000 kgs, so that would pretty much consume his variable reserve.
Realistically, all of this is pretty normal Sydney, so as I see he's simply departed with too little. I guess that's what happens when flight planning departments actually order the fuel.